On switching my main monitor to UWQHD from 1080p, with the integrated monitor on my laptop to the right, this happens. I’m on Plasma 6.4.0 under Wayland. The integrated monitor does reflect that segment of my main screen.
It…fixed itself. Funky…
It showed itself again in 6.4.4.
So you are changing the screen’s resolution, correct? And with that, also the resolution’s aspect ratio from 1920x1080 to 3440x1080, right?
If yes, that means that you have to manually rearrange the screens. I would expect that to happen. Can you click “Edit Arrangement” and move the laptop screen to where it should be? That should solve your overlapping problem.
It can be fixed by manually repositioning the display. Sometimes, they stay lined up correctly, denecessitating the rearangement. Other times not. So my question is as to, because it can be fixed easily, whether to report it as a bug or treat it as a quirk.